Separations of a particulate solid or crystalline phase from a liquid phase by cooling, evaporation, or both are well known. For example, separation of salt from sea water by solar-evaporation may be prehistoric.
Crystallization is important in the preparation of a pure product, since a crystal usually separates out as a substance of definite composition, from a solution or mixture of varying composition. Impurities in the mother liquor are carried into the crystalline product only to the extent that they adhere to the surface or are occluded within the crystals which may have grown together during crystallization.
Crystallization by flash evaporation has also been used to obtain substantially instantaneous decrease in both temperature and pressure, and the attendant substantially instantaneous evaporation of solvent as the relatively hot solution is introduced into the flash crystallization vessel. The rapidly vaporized solvent flashed to the vapor phase permits rapid removal of solvent vapor. Both crystallization and crystal growth occur rapidly with the cooling and concentrating caused by flashing the solution to the lower temperature. Growth of crystals is, substantially, entirely at the lower temperature and is independent of residence time.
Heat exchangers of varying kinds are routinely used in commercial industrial operations. Heat exchangers are used to transfer heat energy from one fluid to another through a heat transfer medium which maintains the integrity of the each fluid stream. In conventional heat exchangers, two fluid streams that remain separate are directed such that heat is transferred from one stream to the other stream through a solid heat transfer medium. The amount of heat transferred between the two fluids depends upon many factors including: the configuration of the heat exchange flow paths, the surface area available to facilitate heat transfer, the heat transfer coefficient of the heat transfer medium, the temperature difference between the exchange fluids, the pressure drop between the inlet and outlet of each fluid stream, and many others.
One example of a typical heat exchanger is commonly called a shell-and-tube heat exchanger. In its simplest form, this type of heat exchanger consists of a small tube disposed within a tube of larger diameter. In the shell-and-tube exchanger, one fluid flows through the smaller inner tube, while the other flows in the annulus created between the outside surface of the smaller tube and the inside surface of the larger tube. Heat is transferred between the two fluids through the smaller inner tube. Accordingly, one fluid increases in energy, while the other decreases in energy. The fluid stream that increases in energy is generically referred to as the "hot side" of the heat exchanger, while the fluid stream that decreases in energy is generically referred to as the "cold side" of the heat exchanger.
Heat exchange between the fluids does not necessary translate into an increase or decrease in the temperature for either fluid stream. Heat exchangers are often used simply to effect a phase change in one of the fluid streams. For example, a heat exchanger may be used to transfer to or from a fluid stream an amount of heat necessary to gasify a liquid stream, or liquefy a gaseous stream, in which case the stream may leave the heat exchanger without a substantial change in temperature.
Traditionally, a major problem in carrying out industrial crystallization by heat exchange, for example in a conventional heat exchanger, is that the crystalline solids deposit on the surface through which heat is transferred, a phenomenon referred to here as fouling. Such fouling reduces the rate of heat transfer and make necessary frequent shutdowns for cleaning of equipment.
Designers of industrial crystallizes have attempted to abate fouling by providing scrapers to continuously remove solid deposits from the heat transfer surface. Such designs typically comprise a set of horizontal jacketed pipes each having a centrally located rotating shaft with peripheral scrapers. Solution of the material to be crystallized is pumped through the inner pipe while cooling fluid is concurrently pumped through the annulus. Heat is extracted from the solution, crystals are formed, and fouling develops on the wall of the inner pipe. Rotating scrapers remove, at least in part, solids from heat transfer surface.
Scrapers have proven to be somewhat effective by increasing run duration from a few minutes or hours to one to fourteen or more days. However, at commercially acceptable heat fluxes the scrapers and rotating shafts themselves become fouled. The solids can become so thick that the inner pipe becomes plugged or the rotating members are damaged. Before this happens it is necessary to shut down the crystallizer and clean the inner pipe, for example by heating it to melt the solid deposits, by washing it with solvent or by manually scraping it.
Other commercially available crystallizers include a set of internally cooled plates disposed in a vertical or horizontal tank. Such designs usually include a rotating shaft to which wipers are attached. Wipers are positioned so that the surfaces of the plates are wiped as the shaft rotates. However, if the temperature differential is increased to provide a good production rate, the shaft, wipers or plates rapidly become fouled requiring an interruption in operation to melt off the solids.
Attempts at improving conventional designs have further comprised heating the scrapers with electric or hot fluid tracers while the scrapers are rotating. Although this can be somewhat effective, doing so usually puts a great deal of heat back into the solution to be cooled and thus limits the capacity of the equipment in addition to increasing operating cost. In any event, the effective installation of heaters on mechanically complicated scrapers is difficult and expensive.
The rate of fouling generally increases rapidly as the difference in temperature between the solution and the cooling fluid increases. As a practical matter, operators of crystallizers generally limit the temperature differential to a magnitude at which they are able to get a fairly long run time between cleanings. However, operating at low temperature differentials requires relatively large surface areas and correspondingly large capital investment to provide commercially acceptable capacity. Accordingly, the ability to use higher temperature differentials would likely result in a substantial reduction in crystallizer capital cost.
British patent No. 1,365,536 discloses a counter-current crystallizer apparatus which comprises individual crystallization, purification, and melt sections. Each purification section comprises a plurality of perforated plates positioned at spaced intervals in a cylindrical enclosure so that the crystal mass may pass the plates counter-currently to the mother liquor. Free moving bodies, such as spheres, are placed on each perforated plate. The spheres are set in motion by vibrating the entire column, vibrating the set of perforated plates or by other means. Although this invention does increase the purification efficiency, it still suffers from the short run time characteristic of conventional crystallizers because this invention incorporates conventional chillers to generate the crystal crop that is subsequently purified in the above-described purification section, and crystallization in tubes tends to cause plugging.
U.S. Pat. No. 4,981,190 to Carter et al. describes a type of crystallizer having a plurality of vibrating perforated plates at intervals along the crystallizer length. More particularly the patent discloses a crystallizer column including a plurality of substantially horizontal perforated plates periodically attached to a central shaft located coaxially within a normally elongated housing. A plurality of heat transfer tubes extend along the axial length of the column through apertures fabricated in the horizontal plates. Mobile bodies substantially cover the surface area of each perforated plate. A material excitation device which is said to produce two waveforms is attached to the central shaft. Frequency waveform that results in the mobile bodies colliding with one another, the inner surface of the enclosure and the surface of the heat transfer tubes. The second waveform is a high amplitude, low frequency waveform that causes the plates to move along the length of the heat transfer surfaces so that the surfaces are scraped of any fouling that occurs.
U.S. Pat. No. 5,523,064 to Schranz describes a surface-cooled fluid bed crystallizer apparatus using submerged heat exchanger surfaces (typically heat exchanger tubes or plates through which a coolant is passed) which surfaces are bathed with a stream of gas bubbles (preferably air). Gas bubbles are said to increase localized velocity at the heat exchanger surfaces, improve heat transfer, reduce crystallization on the heat exchanger surfaces and gently keep crystals in suspension, thereby avoiding unwanted nucleation as is characterized by the use of mechanical circulation devices. Operation requires, however, continuous recalculation and the removal of depleted magma and crystals, in a continuous mode.
Laboratory evaluations indicate that heat transfer surfaces in crystallizers can be kept clean by inducing ultrasonic vibrations in them. This method does in fact work well in laboratory apparatus. Unfortunately, no way has been found to scale up ultrasonic crystallizer units to a commercially acceptable capacity.
Commonly assigned U.S. Pat. No. 3,497,552 to Olsen discloses that purification of impure organic materials dissolved in a liquid, such as water, can be accomplished by continuous crystallization in a plurality of series connected cooling stages. The cooling stages are facilitated by the addition of cooled solvent at each stage. Unfortunately, this requires extensive solvent handling which is generally undesirable.
An additional barrier to the use of heat exchangers for crystallization, has been the tendency of the crystalls to accumlate within the heat exchanger and thereby plug the heat exchanger flow. This could result in a dangerous situation because of pressure build-up caused by the plugging which might lead to a violent rupture.
It is therefore a general object of the present invention to provide an improved process for phase separating, from a solution or mixture, a material present therein by crystallizing it through heat exchange which overcomes the aforesaid problems of prior art methods.
It is another object of the present invention to provide a method for selectively crystallizing, from a fluid mixture or solution a material present therein, the chemical material by continuous heat exchange in a plate heat exchanger thereby forming a slurry comprising crystals of the chemical material and a resulting mother liquor.
It is yet another object of the present invention to provide a method of crystallization of para-xylene, dimethylnaphthalene and aromatic carboxylic acids and anhydrides which reduces the necessary equipment and reduces overall solvent handling compared to prior art processes.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and appended claims.